Thermal Decomposition of Hydrazine Vapor in Silica Vessel T. J. HANRATTYl, J. N. PATTISON, AND J. W. CLEGG Battelle Memorial I n s t i t u t e , Columbus, Ohio
A. W. LEMRION, JR. T h e Ohio S t a t e University, Columbus, Ohio
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WING to its high posiLive free energy of f o r m a t i o n , h y d r a z i n e is potentially unstable even at room temperature. Equilibrium constants and enthalpy changes for a number of pos&ble modes of decomposition are presented in Table 1. F~~~ these data it may be concluded that complete decomposition of hydrazine is possible at all temperatures,
and its decomposition will be accompanied by the Of about 30 kg.-cal. per mole. I n their studies of the rate of thermal decomposition of hydrazine below 340' C., Askey (1)and Brown ( 4 )experienced difficulty in reproducing results from different runs, as well as in obtaining consistent data in a single run. However, they indicated that the decomposition approximates a first-order reaction mechanism and that silica and borosilicate glass surfaces decompose hydrazine. More recently, data have been obtained b y Szwarc ( I S ) on the heterogeneous reaction from 621" to 794" C. in the presence of a large excess of toluene. However, no attempt was made to determine the effect of the toluene upon this reaction. I n view of the uncertainties in available data and the limited temperature ranges covered, the results of this investigation are presented* Its scope Of a study Of the rate Of position of hydrazine in a silica reactor from a hydrazine-water mixture containing 95.5 weight %hydrazine over the temperature range 269 O to 637' C. and at surface-to-volume ratios of 2.5 and 18 cm-l. METHOD OF ATTACK
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Investigating such a rapid and exothermic reaction over such a range of variables required an extremely flexible apparatus. The apparatus shown in Figure 1 was designed to give this flexibility with as little sacrifice of accuracy as possible. The hydrazine was admitted to the system by a buret designed to give a constant feed rate. This was accomplished by bubbling gas from a constant-pressure source into the bottom of the buret, thus maintaining a constant delivery pressure. By the use of removable capillary tips on the bottom of the buret, it was possible to vary the hydrazine flow rate over wide limits. The hydrazine was carried through a vaporizer by a stream of argon gas. Another stream of argon gas amounting to about three fourths of the total was passed over a heating element and preheated to a temperature slightly above the control temperature in the reactor. The investigation covered almost a thousandfold variation of the rate of decomposition, the necessary variation of oontact 1 Present address, Department of University, Princeton, N. J.
time being obtained by varying the rate Of argon flow* Flow rate was controlled manually with the aid of two capillary flowmeters. B~ the use of removable flow tips, i t was possible to cover the necessarily large flow range. By proper adjustment of the relative rates of hydrazine and argon flow., i t was possible to maintain the fraction of hydrazine in the gas mix-
This work was undertaken to get a better insight into the rate and mechanism of hydrazine decomposition as a basis for the study of its prevention. Hydrazine decomposition was studied in a flow system highly diluted with argon to absorb the heat of reaction. The experiments were conducted in a straight silica tube both empty and filled with silica rods, in order to determine wall effects. The temperature range covered was from 269' to 637" C. Under the conditions tested, the decomposition was found to be first order and heterogeneous. The rate of hydrazine decomposition in a silica vespel was found to be sensitive to the source and history of the surface employed. These results indicate that it may be possible to decrease hydrazinedecomposition at these temperatures if smaller surface-to-volume ratios are used. The use of surface coatings or different reactor materials may also help.
Chemical Engineering. - Princeton
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ated by hydrazine decomposition was absorbed by the large excess of argon gas and the temperature control in the reactor was not appreciably disturbed. An enlarged view of the silica reactor is shown in FigThe reaction zone was arbitrarily selected as the 15-cm. ure 2. length shown. Surface-hvolume ratio was changed by packing the reactor with 17 silica rods approximately 15 em. in len t h and 4 mm, in diameter, The ends were fire-polished to avoid passibility that the sharp edges might catalyze the reaction better than smooth surfaces. Because of their regular shape, i t was possible to calculate the increase in surface area with precision. No cleaning or preheating was given to the silica used for the reactorand EXPERIMENTAL ERROR
The two largest sources of error in the experimental procedure
were attributed to end effects in the reactor and the difficulties of precise temperature measurement, The first of these was partially alleviated b y using a preheating section to bring the hydrazine to temperature as soon as it entered the reactor proper and by adjusting the position of the reactor in the furnace so that cooling of the exit gases was very rapid. The effectiveness of this method can be seen b y examining the temperature traverse in Figure 3.
DECOMPOSITION TABLE I. MODESO F HYDRAZINE Decomposition t o Ammonia and Nitrogen
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3Ni"(g) = 4NHa(g) %(e;) K p a t 300° K. = 1 5 X l o g 4
K p a t 1000a K. = 8 . 9 X 10*6 A H o = -37,470 cal./mole a t 298O K.
Decomposition to Nitrogen and Hydrogen
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NzHAg) = N z k ) 2HzM K p a t 300° K. = 4 . 9 X 1 0 2 7
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K p a t 1000° K. = 4 , 2 X -22,750 cal./mole a t 298' K
Decomposition t o Nitrogen, Ammonia, and Hydrogen 2NrHdg) = 2NHs(g) C NaW C H z k ) Kp a t 1000° K. = 5.3 X 10zs Kp a t 300° K. = 8.6 X 1060 AHo -33,790 oal./mole a t 298O K. 3NzHi(g) = 2 N H s k ) 2Ndg) + E 3 ~ 3 ~ 0K. 0 0= 2 , 2 104a Kp a t 300' K. = 4 . 2 X lo8* A H o = -gO,llO cal./mole a t 298O K. Data calculated from Scott el at. for hydrazine (II), Stephenson and McMahon for ammonia ( l a ) , and Krieger for nitrogen and hydrogen (8).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 43, No. 5
CORRELATION OF DATA
ARGON GAS-, +DELIVERY
BURET
The data were correlated by assuming the rate of decomposit,ion to be directly proportional to the amount of hydrazine in the gas phase. (Justification for this assumption appears later in the article. )
OVABLE GAS FLOW TIPS
d_n - 7L.n REMOVABLE FLOW TIP
dt
where n = moles of hydrazine t = time k = proportionality constant
-GAGE CHROMEL-A FLOW METERS CONTAINING Dl8UTYL PHTHALATE
THERMOCOUPLE WELL
For a constant-temperature flow process in which longitudinal diffusion and the change in volume as the reaction proceeds are negligible, the above equation may be integrated to yield : X:=-]n-1 no V R / & n1
DISTILLED
H20
Figure 1. Diagram of Apparatus
The temperatures observed were subject to the errors inherent in.the measurement of temperatures of a floving gas ( 3 , 9 ) . Temperature measurements were conducted by inserting a bare 28gage Chromel-Alumel thermocouple, wrapped with aluminum foil, directly into the gas stream before admitting hydrazine. The use of aluminum foil minimized errors in temperature measurement due to radiation from surrounding surfaces, and the small cross-sectional area of the temperature probe minimized errors due to heat conduction. The absolute accuracy of the temperature measurement did not justify accounting for the variation in temperature along the reactor length. Therefore, an integrated average was calculated and this was used in subsequent correlation.
INLET TUBE 3.5 C
T
THERMOCOUPLE WEL
0.D.:0.725 CM.
OUTLET VOLUMEz7.6 CC.
ANALYSIS O F REACTION PRODUCTS
The recovery system consisted of four Fisher-Milligan gas mashers containing water and sulfuric acid, The contents of the Figure 2. Diagram of Reactor scrubbers were analyzed for hydrazine with potassium iodate, as described by Penneman and Audrieth (10). Free base or unreacted acid \vas deTABLE 11. DATAON UNPACKED REACTOR termined with either standard hydroSurface area = 1 3 8 . 1 s q . em. chloric acid or sodium hydroxide soluVolume = 5 5 . 0 eo. tion, using methyl red as the indicator. Surface-to-volume ratio = 2 . 5 cm. -1 The amount of ammonia in the deInlet composition products was equal to the Sd34 Concn. a t total free base minus the hydrazine, SonconReactor densable Reactor as determined by potassium iodate Average PresTime Gas Inlet Outlet Outleta CondiContitration. Reactor sure of Flow, SzH?, N2H4, hH3, tions tact Velocity The hydrogen content of the gases R u n Temp., Mm; Run, Std. Milli- MilliMilliMillil Time, Constant, emerging from the last scrubber was No. .a C. Hg Min. Cc./Min. moles moles moles moles/L. See. k, Sec.-1 determined for some of the runs by 2 391 772 179 2280 800 167.6 0.79 0.57 0.55 4 484 841 79.7 3190 960 260 1.36 0.37 3.4 burning a volume of the gas over a 5 496 837 151.9 3340 764 146.7 763 0.56 0.37 4.5 platinum wire and measuring the con6 590 831 140 3280 916 56.1 1090 0.65 0.33 8.5 traction in volume. However, because 7 637 829 160 3730 995 17.4 1176 0.52 0.27 14.8 9 340 789 216.3 741 270 135.1 186.9 0.74 1.97 0.35 the hydrogen was such a small fraction 11 301 789 65 478 343 314 17 4.3 2.7 0.033 of the gases leaving the reactor, these 12 493 806 100 2500 317 54.1 334 0.46 0.48 3.7 14 548 803 250 3240 239 7 . 3 317 0 , 1 0 3 0 . 3 5 9.8 t e s t s w e r e o n l y qualitative. Nitrogen and hydrogen in the reaction Total base in products minus hydrazine recovered. Nitrogen carrier gas. Incomplete ammonia recovery, no acid used in recovery system. products were determined by material balances.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
where V R = volume of the reactor Q = volumetric flow rate no = moles of hydrazine entering the reactor nl = moles of hydrazine leaving the reactor
TABLE111. DATA ON
PACKED
1115 REACTOR
Surface area = 441.6aq. om. Volume = 24.1 00. Surface-to-volume ratio = 18 om.-' Reactor Average Pres- Time Inlet Outlet Outleta Reactor aure, of NsH4, NzH4 NHa, Run Tsmz., Mm. Run, MilliMillil MilliNo. Hg Min. moles moles moles 15 473 835 240 134 40 6 90 16 386 831 65 589 218.0 483 170 276 19.0 309 17 433 845 18 433 837 110 186 11.0 210 19 409 827 200 263 38.2 295 20 357 827 245 298 92.4 258 21 499 829 340 180 6.3 214 22 269 779 110 312 106 0 271 23 388 773 140 339 57.5 356 a Total base in products minus hydrazine recovered.
Values of k have been calculated from this equation and are presented in Tables I1 and 111. The variations of the proportionality constants with temperature have been correlated by the Arrhenius plots (6) in Figure 4, in which the positions of the lines drawn through the points were calculated by the method of least squares.
Inlet NzHa Noncon- Concn. at densable Reactor Gas Flow, Conditions, Contact Std. MiiliTime, Cc./Min. moles/L. Sec. 5000 0.045 0.116 3740 1.04 0.166 4800 0.145 0.133 4890 0.146 0.125 4880 0.116 0.128 4820 0.118 0.146 4910 0.042 0.113 1680 0 84 0.42 7370 0.137 0.081
Velocity Constant, k, Sec.-I 10.3 5 2 20 23 15.1 8.0 30 2 5 22 0
RATE OF DECOMPOSITION ?a
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The data on the unpacked reactor represent a 42-fold variation in the inlet hydrazine concentration and a 10-fold variation in contact time. Those for the packed reactor represent a 25-fold variation in inlet concentration and a 5-fold variation in contact time. The deviations of these points from the straight line drawn by least squares are within the experimental error. Correlations based on second- and third-order reaction mechanisms produce curved lines. Therefore, the equation used to define the rate of hydrazine decomposition appears to correlate the data satisfactorily. Two runs were made with nitrogen gas. No significant difference was noted between these runs and those in which argon was used. These indicated that the inert carrier gas acted merely as a diluent and did not affect the rate of reaction. The heterogeneity of the reaction is demonstrated by the large increase in reaction rate brought about by increasing the amount of surface available for reaction. From the slope of the two lines in Figure 4, the energy of activation for the unpacked reactor was calculated t o be 15.7 kg.-cal. per mole and that for the packed reactor, 9.3 kg.-cal. per mole. Values of this magnitude are typical of heterogeneous reactions (14). Inasmuch as no sharp breaks are evidenced in the curve, decomposition is occurring predominantly on the surface. Homogeneous reactions are characterized by high activation energies and, consequently, a very rapid increase of the reaction rate with temperature. Therefore, the initiation of reaction in the gas phase would markedly increase the slope of these graphs. Velocity constants calculated from the data of the first two runs for the packed reactor fall far below the line representing the data for this system. Run 21 was conducted a t approximately the same operating conditions as run 15. The large difference in the velocity constants indicates, however, that the observed phenomena were not peculiar t o the conditions a t which the early runs were carried out, other than the possible lack of conditioning or activation of the rods. Therefore, it appears that the sur-
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face must undergo a certain amount of change before consistent results are obtained. Similar results have been reported in studies of the thermal decomposition of ammonia (6). Further evidence for the sensitiveness of hydrazine decomposition t o the history of the surface is found by comparing the data of the runs for the packed and unpacked reactors. A 7-fold increase in the surface-to-volume ratio, instead of causing a proportionate increase in the rate of reaction, produces 8- to 23-fold increases. This, in addition to the fact that different activation energies were obtained, indicates that the surface of the packings was different from the surface of the reactor, despite the fact t h a t they were both silica. This difference may be due to differences between the past histories of the reactor and the rods or to surface impurities on one of them. EFFECT OF DIFFUSION RATE
Analysis of the data presented has shown that the hydrazine decomposes entirely on exposed silica surfaces. Before reaction occurs, the hydrazine must, therefore, diffuse to an exposed surface. Owing to the very rapid reaction occurring, there is a possibility that the rate of diffusion of hydrazine to the wall is controlling the rate of decomposition. I"
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3 4 5 6 7 8 9 0 1 1 12 I 3 1 4 1 5 DISTANCE FROM TOP OF REACTOR, CM.
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Figure 3.
Reactor Temperature Traverse
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Figure 4. Arrhenius Plots of Hydrazine Decomposition Data
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Hulburt (7) presents an integrated expression for a first-order reaction on a cylindrical surface sparsely covered by the reactants and producbs. For a process in which the rate of diffusion of the reactant to the wall is very slow in comparison to the rate of reaction on the wall, his derivation reduces to: no
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